By the early years of the space age, scientists had come to understand that tektites are ejecta from meteorite impacts, but a debate ensued over whether those impacts had occurred on the Earth or on the Moon. Once scientists had analyzed the Apollo samples, it was clear that their chemical compositions ruled out the Moon as the tektite source. Scientists have searched without success for the crater from which the Australasian tektites came, and just as this book is being written, reported indirect evidence of its presence underneath young volcanic rocks in Southern Laos.
Two other examples of tektite-like objects are the Libyan Desert Glass and the Dakhleh Glass of Western Egypt. These two are regarded as due to an ET event, yet no impact craters have been found. In these cases, Boslough and colleagues concluded that “these enigmatic impact glass occurrences—for which no craters have been identified—may be the result of airburst events.” Just what the YHIH proponents say happened at the YDB.
But how common might such events be? W. M. Napier modelled the disintegration of a large comet, using as an archetype Comet Encke, a 5-km (3 mi) body that orbits the Sun every three years. (Some scientists believe that the Tunguska object was a fragment of Comet Encke.)
Napier found that the material from a disintegrating comet would spread in a few days over a range wider than Earth, making the likelihood of an encounter with the cometary debris much greater than a collision with the comet’s core itself. The result would be a short-lived cometary “hurricane” more intense than anything in modern experience. The resulting smoke and debris could block sunlight and cause brief periods of cooling, such as occurred during the YD. Napier concluded that “Such an encounter may have contributed to the large animal extinctions and sudden climatic cooling of 12 900 yr ago.”
The following summary, modified from Wolbach I and Moore et al. on Abu Hureyra, presents the latest thinking of scientists researching the YDIH:
About 12,800 years ago, Earth collided with the immense debris field of a fragmented giant comet that resembled today’s Comet Encke. This gave rise to hundreds or thousands of Tunguska-like airbursts and impacts across at least four continents in the Northern and Southern Hemispheres, an area half the size of Earth. A hyper-velocity plume of vaporized cometary materials and platinum-group element (PGE)–rich target rocks injected Pt, Ir, Os, and other heavy metals into the stratosphere accompanied by impact-related nanodiamonds, meltglass, and microspherules. At some sites, the airburst created a ground-hugging blast wave that carried molten pieces of glass horizontally several kilometers from ground zero.
The impact event destabilized the ice-sheet margins, causing extensive iceberg calving into the Arctic and North Atlantic Oceans. The air-bursts and impacts collapsed many of the dams that were impounding proglacial lakes along the ice-sheet margins, producing extensive meltwater flooding into the Arctic and North Atlantic Oceans. The massive outflow of proglacial lake waters, ice-sheet meltwater, and icebergs into the Arctic and North Atlantic Oceans caused rerouting of oceanic thermohaline circulation. The resulting climatic feedbacks led to the YD cool episode. Warm interglacial temperatures abruptly fell to cold, near-glacial levels within less than a year, possibly in as little as three months. A rapid increase in wind strength across Greenland at the YD onset deposited extensive dust, sea salt, Pt-rich impact debris, and combustion aerosols onto the ice sheet.
The radiant and thermal energy from multiple explosions triggered wildfires that burned ∼10% of the planet’s biomass, as evidenced by anomalously high charcoal peaks. The widespread biomass burning generated large amounts of long-lived, persistent AC/soot that blocked nearly all sunlight, rapidly triggering an impact winter that transitioned into the YD cool episode.
Climate change, wildfires, and related environmental degradation contributed to the late Pleistocene megafaunal extinctions and human cultural shifts and population declines.
The reproducibility of the evidence has moved the YDIH much closer to the status of theory than hypothesis and shifted the burden of proof to its opponents. This allows us to ask what role the impact might have had in the disappearance of the Pleistocene megafauna and the Clovis culture.
PART III
EFFECTS
14
THE ENIGMA OF THE YOUNGER DRYAS
In 1990, W. H. Berger reviewed the possible causes of the YD and concluded, “The origin of the Younger Dryas is likely to remain an enigma for some time to come, perhaps forever. If the cold spell resulted from an interplay of positive feedback mechanisms within the climate system, it will not be possible to distinguish cause and effect.” This even though, as Broecker wrote in 2010, “No event in the climate record has received more attention than the Younger Dryas.”
To illustrate the conundrum, let us follow Broecker’s writings through the years. In a 1989 paper, he, James Kennett, and others found evidence for a proposal that attributed the trigger of the YD cooling to the diversion of glacial meltwater from the southward flowing Mississippi River drainage to the eastward flowing St. Lawrence system, as shown in the map below. Geologists refer to the change as “the great plumbing shift.”
When the fresh glacial meltwater reached the North Atlantic via the St. Lawrence, instead of mixing with the salty and more dense seawater, it floated on top. Soon to join it was the vast amount of freshwater released by the collapse of giant glacial Lake Agassiz. The surface layer of low-density freshwater blocked the circulation of the Atlantic Ocean itself. Normally, surface currents like the warm Gulf Stream flow poleward from the equator, cooling and growing denser as they go, until in the vicinity of Iceland they sink. This water then flows southward at depth into the Southern Ocean, then northward again, as part of what has been called the ocean conveyor belt. Broecker, who sadly died as this book was being written, and others proposed that the arrival of the buoyant meltwater could have shut down the conveyor and left the cool, freshwater sitting on top, to chill the adjacent continents and bring on the YD. Maintained by feedbacks, the cold period lasted until the conveyor belt circulation was re-established approximately 1,300 years later.
Lake Agassiz is the linchpin of this hypothesis. Larger than the present Great Lakes combined, it stretched across much of Manitoba, Saskatchewan, northwestern Ontario, as well as parts of Minnesota and North Dakota. At its height, it was larger than any freshwater lake today.
Lake Agassiz originally drained down the Mississippi system, but some researchers proposed that when the ice dam that had impounded it melted away, the lake’s northern and eastern shorelines opened, sending an enormous volume of lake water eastward into the St. Lawrence. More recently, other scientists have shown that some of the water from Lake Agassiz traveled north down the Mackenzie River and into the Arctic Ocean, near the start of the Younger Dryas.
FIGURE 17:
Upper North America at the End of the Pleistocene. Arrows show drainage directions; blue represents the pro-glacial lakes.
In a 2006 article in Science titled, “Was the Younger Dryas Triggered by a Flood?” Broecker reported that aerial searches between Lake Agassiz and Lake Superior had failed to find the flood channels and boulder fields that the southeast-flowing floodwaters from Lake Agassiz should have left as evidence. This led him to consider “alternative triggers for the Younger Dryas cold episode,” but he could find none that worked. This seemed to return the question of the origin of the YD to the state that Berger left it: “In the end, the initiation of the Younger Dryas may remain a mystery.”
One issue facing anyone attempting to explain the YD is that during the Pleistocene, one that we have touched on, there was not just one abrupt temperature change but some two dozen. Was the YD just another of these oscillations and thus an expected if not fully understood aspect of deglaciation, or was it somehow fundamentally different? By 2006, Broecker, leaning in Berger’s direction, answered: “The Younger Dryas is unique to the termination of the last glacial cycle. Hence, the YD was likely triggered by a freak event rather than by something common to each
glacial termination.”
Then, in the 2010 article, “Putting the Younger Dryas into Context,” Broecker had changed his mind. The abstract is worth quoting in its entirety:
The Younger Dryas event is by far the best studied of the millennial-scale cold snaps of glacial time. Yet its origin remains a subject of debate. The long-held scenario that the Younger Dryas was a one-time outlier triggered by a flood of water stored in proglacial Lake Agassiz has fallen from favor due to lack of a clear geomorphic signature at the correct time and place on the landscape. The recent suggestion that the Younger Dryas was triggered by the impact of a comet has not gained traction. Instead, evidence from Chinese stalagmites suggests that, rather than being a freak occurrence, the Younger Dryas is an integral part of the deglacial sequence of events that produced the last termination on a global scale.
The article concluded, “There is no need to call upon a one-time catastrophic event to explain the YD. More likely, the YD was a necessary part of the last termination.” Yet, to have the solution to the great mystery rest on a few stalagmites in caves halfway around the world seems less than satisfying. One suspects that Broecker himself may not have been fully content with this explanation.
In any event, as of 2010, despite years of concentrated study, the best that scientists led by Broecker could come up with was that the YD was just a natural consequence of the end of the last Ice Age. No freak event, no impact, necessary. This is reminiscent of the reaction of many vertebrate paleontologists to the Alvarez Theory, expressed by one this way:
A satisfactory explanation of the cause of the extinction of the dinosaurs has been known for some years. Probably more than 99.99999% of all the species that have ever existed on the Earth are now extinct. The dinosaurs are among these. Extinction is the normal way of life. As far as is currently known, it does not seem necessary to invoke an unusual event to account for the demise of the dinosaurs.
Geologists do not like to have their long-standing mysteries solved by a deus ex machina, a proverbial bolt from the blue.
Then came the 2013 discovery of the Pt spike at the YDB, and Broecker changed his mind again. Had he been a politician, and thankfully for science he was not, his opponents would denounce him as a flip-flopper. But instead he was that rare scientist who, when presented with new evidence, is willing to change his mind and say so. In the history of the great scientific controversies in the earth sciences — continental drift, meteorite impact, the Alvarez Theory, and anthropogenic global warming — you can count on your digits the prominent scientists who ever publicly changed sides.
But Broecker never went all the way with the YDIH, writing in his white paper that, “Although I don’t for a minute believe that this impact did in the mammoths and the Clovis people, I do think that it triggered the YD.” On the other hand, he could not “buy it’s a coincidence.” His last word on the subject was, “The only way I can rationalize these three events is to view the system to be approaching instability in which case, a small jolt of noise pre-triggers a change which was due to occur spontaneously.”
Broecker cites only the finding of the Pt spike in the Greenland ice core, showing that he evidently wrote before Moore et. al (2017) reported that the Pt spike is widespread. Broecker’s white paper shows him continuing to wrestle with the great question that had puzzled him for so long. We see him coming right to the edge of attributing the Clovis decline and megafaunal extinction to impact, then stepping back to say that they would have happened anyway. It seems unlikely that he was satisfied with this explanation either. In any case, if impact “triggered the YD” and provided “a small jolt of noise” that spurred the Clovis decline and mammal extinction, then it was partly responsible for those events. But maybe this is splitting hairs. As to what Broecker would have made of all the evidence that has come to light by the time this book is being written, we cannot know. But we can be sure that he would have considered that evidence carefully and followed where it led. He knew better than almost anyone that decades of research had failed to solve the trifold mystery of the YD cooling, the megafaunal extinction, and the Clovis decline. His mind would have been open to a new hypothesis and so should ours be.
15
CLOVIS
In the original YDIH article, the FEA authors hypothesized that an ET event had “contributed” to the cooling, the extinction, and the Clovis decline. This use of “contributed” reminds us of the way that climate scientists today, rather than saying that anthropogenic global warming “caused” the latest of the rising number of extreme weather events, instead ask whether it has made such events more likely — increased the odds if you will.
We know that Homo sapiens survived the YD in North America and that the distinctive artifacts of the Clovis people, exemplified by their beautiful, fluted stone points, did not. Thus, instead of looking for something analogous to species extinction, we are interested in whether this cultural shift happened so rapidly and was so profound that it likely had an external trigger. Before focusing on the end of Clovis, let us turn to its beginning, so that we can better appreciate what was lost.
FIGURE 18:
Clovis Points
CAREER KILLER
North American archeology is a surprisingly young field, or perhaps not so surprising when one considers how recently Europeans settled parts of the vast North American continent. Then too, the radiocarbon revolution did not arrive until the 1950s and it took several more decades for scientists to overcome its inherent problems and begin to use the more precise accelerator mass spectrometer.
What we might call the founding Clovis discovery occurred in 1908 when one of those seemingly fictional Old West characters happened to be patching fence near small Folsom, New Mexico after a flood. George McJunkin was born into slavery in Texas and after emancipation went on to teach himself to read and write, speak Spanish, play musical instruments, and become an expert cowboy and “amateur geologist, astronomer, and natural historian.” He had his own specimen collection, built up through many long but attentive hours in the saddle.
Exposed by the flood in a small canyon where he was walking, McJunkin saw several huge bones sticking out, so large that they could not have come from any known creature. He tried to interest people in his discovery, but only a local blacksmith took the trouble to visit the site. Four years after McJunkin’s death in 1922, the blacksmith brought the find to the attention of Jesse Figgins, director of the Colorado Museum of Natural History (now the Denver Museum of Nature & Science). Figgins sent a team to excavate the bones, which turned out to be from the nearly eight-foot tall Bison antiquus, one of the ice-age species that had gone extinct at the YDB, our smaller, modern Bison having survived. The excavators soon found a pair of hand-crafted spear points of a type not seen before. Part of one point lay amidst the bones, showing that they were contemporaneous. Since Bison antiquus was known to have been extinct for thousands of years, the people who crafted the spear points must have also lived thousands of years ago. If this reasoning was correct, then skilled human hunters had co-existed with pre-historic animals.
As author Charles Mann writes in 1491, Figgins was both “intrigued and dismayed.” Dismayed because even though he was not an archeologist, Figgins knew that the notion that humans had existed in North America for thousands of years before Europeans arrived was anathema to archeologists. And for amateurs to claim to have uncovered the purported evidence only made matters worse.
One of the first to discover ancient artifacts in North America was just such an amateur — and a troublesome one at that. In 1872, a New Jersey doctor named Charles Abbott discovered arrowheads, hide scrapers, and axheads on his farm. Other amateurs found similar objects at several locations in the East and Midwest. Religious leaders rejected these claims of antiquity because they contradicted the Biblical story of creation. Because the bones contradicted scientific orthodoxy, the scientists of the Smithsonian Institution’s Bureau of American Ethnology also rejected them. When both the
Bible and science are against you, you don’t get a third strike.
The Smithsonian sent one of its most distinguished scientists to investigate the claims of Abbott and others. William Henry Holmes was an “American explorer, anthropologist, archaeologist, artist, scientific illustrator, cartographer, mountain climber, geologist and museum curator and director.” He holds a special place among geologists, having been a member of both the Hayden and the Dutton pioneering surveys of the Grand Canyon.
Holmes examined several sites where the purported ice-age artifacts had been found, including Abbott’s own farm. He reported that they were not ancient after all, but merely the discards of Indians from the American colonial period, due not to antiquity but to “two hundred years of aboriginal misfortune and Quaker inattention and neglect.” (Abbott was a Quaker.) Holmes’s successors at the Smithsonian continued to reject any and all reports of ancient Indians, to the point that for any archeologist to claim otherwise was a “career-killer,” in the words of a subsequent director of the Bureau of American Ethnology. The leader of this school of rejection was a man of Bohemian ancestry named Aleš Hrdlička, who also was suspicious of “female scientists, genetic analysis, and the entire discipline of statistics."
By 1928, Hrdlička had come to accept the presence of “ancient man” in Europe, because there human artifacts are present, as he said, in “such large numbers that already they clog some of the museums.” But, he asked, “Where are any such things in America? Where are the implements, the bones of animals upon which these old men have fed? ... Where is the explanation of all this? What is the matter?” Several months before, Figgins had met with Hrdlička and Holmes to show them the Folsom spear points, which should have answered Hrdlička’s questions. But no, Hrdlička continued to deny any evidence of Pleistocene man in the Americas, never changing his mind. He traveled down Robert Park’s road of science to a fork, and chose the route that led to the dead end of denial.
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